IP Multimedia (IPMM) services provide a dynamic combination of voice, video, messaging, data, etc, within the same session. By growing the numbers of basic applications and the media which it is possible to combine, the number of services offered to the end users will grow, and the inter-personal communication experience will be enriched. This will lead to a new generation of personalised, rich multimedia communication services, including so-called “combinational IP Multimedia” services.
IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks. IMS provides key features to enrich the end-user person-to-person communication experience through the integration and interaction of services. IMS allows new rich person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network. The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Whilst SIP was created as a user-to-user protocol, IMS allows operators and service providers to control user access to services and to charge users accordingly. Other protocols are used for media transmission and control, such as Real-time Transport Protocol and Real-time Transport Control Protocol (RTP/RTCP).
Within an IMS network, Call Session Control Functions (CSCFs) perform processing and routing of signalling. CSCFs handle session establishment, modification and release of IP multimedia sessions using the SIP/SDP protocol suite. 3GPP TS23.228 describes the logical nodes P-CSCF, I-CSCF, S-CSCF, E-CSCF and BGCF. The S-CSCF conforms to 3GPP TS 24.229 and performs session control services for User Equipments (UEs). It maintains the session state to support the services, and performs the following functions:                it acts as a registrar according to [RFC3261] at registration;        it notifies subscribers about registration changes;        it provides session control for the registered users' sessions;        it handles SIP requests, and either services these internally or forwards them on to a further node; and        it interacts with IMS Application Servers.        
The S-CSCF performs SIP routing according to 3GPP routing procedures. For inbound SIP traffic, the S-CSCF will route sessions to that P-CSCF whose address was stored during subscriber registration. For outbound SIP traffic, the S-CSCF interrogates a DNS/ENUM to determine how the call should be routed. The S-CSCF interacts with the Home Subscriber Server (HSS) to obtain subscriber data and to exchange authentication information using DIAMETER messages.
A User Equipment (UE) generally connects to an IMS network through a Radio Access Network (RAN). The first point of entry to the IMS network is a Proxy Call Session Control Function (P-CSCF). Once a UE is registered with a particular P-CSCF, all the SIP signalling originating and terminating at the UE will be routed through this P-CSCF.
Existing radio access networks comprise cellular networks according to the 2G and 3G standards. The process of rolling out so-called 4G networks has just begun, and it will be many years before 4G network coverage is sufficient to allow 2G and 3G networks to be withdrawn completely.
Considering further the 4G technology, this is being specified under the name LTE (Long Term Evolution) and SAE (System Architecture Evolution) in 3GPP. The LTE radio access network technology implements only a packet switched access, in contrast to 2G and 3G (using GERAN and UTRAN radio access network technologies respectively) which provide for both packet switched and circuit switched access. In 2G and 3G networks, packet switched connections are used to carry data whilst circuit switched connections are used for real-time services such as voice calls. In 4G networks, all services will be carried via packet switched connections. In the case of a voice call initiated when a user is attached to an LTE radio access network (termed Enhanced UTRAN or E-UTRAN), that call will of course make use of a packet switched connection.
The IMS network may comprise several Application Servers (AS), which implement a variety of services. For example, a voice call may be established using the IMS network by a Multimedia Telephony (MMTel) Application Server, which implements service logic for establishing and controlling voice calls.
Specifically, the GSM Association (GSMA) has published the Voice over LTE (VoLTE) profile, (GSMA IR.92 IMS Profile for Voice and SMS V4.0 http://www.gsmworld.com/documents/IR9240.pdf). It specifies the network and terminal interoperability requirements for a telephony service over an LTE RAN, using IMS as the telephony service engine.
In case of a failure of a P-CSCF with which a UE was registered, several situations may arise. For example, if the UE wishes to start an originating communication leg that is handled by an AS in the IMS network, such as for example a packet voice call, the UE will not receive a valid answer from the P-CSCF, which is unavailable. While the UE cannot reach the IMS, in such case the UE is configured to register with a different, available P-CSCF node. Once re-registration has occurred, the UE has restored the access to the IMS network and may place the originating call through one of the Application Servers thereof.
If the UE is at the terminating end of a communication chain that is being handled by an AS in the IMS network, such as for example a packet voice call, the following problem arises. An S-CSCF node in the IMS network directs a SIP INVITE request to the P-CSCF with which the target UE was registered. The P-CSCF should usually extend the request to the UE, so that the UE is able to terminate the communication leg. However, if the P-CSCF is unavailable, the requesting S-CSCF has no means of contacting the UE, and the call fails. While this problem arises for calls in a VoLTE setting, other IMS based services trying to reach the UE through the failed P-CSCF are affected in the same way.
FIG. 1 schematically illustrates the architecture under consideration for a VoLTE setup. A user terminal, or UE, connects to an LTE radio access network. The enhanced Node B (eNodeB) provides inter alia control of radio access within the LTE RAN. The serving gateway (S-GW) sits in the user plane where it forwards and routes packets to and from the eNodeB and the Packet Data Network (PDN) Gateway (P-GW). The P-GW node interfaces with other packet data networks, such as for example the depicted IMS network, of which the illustrated P-CSCF node is the entry point. The UE registers with a P-CSCF node. FIG. 1 further illustrates a Policy and Charging Rules Function (PCRF) node and a Mobility Management Entity (MME). The S/P-GW and the MME reside in the LTE Evolved Packet Core (EPC). The MME is the key control node for the LTE access-network. It is responsible for idle mode UE tracking and paging procedures including retransmissions. FIG. 1 further shows a Home Subscriber Server (HSS) that resides within the subscriber's home network.
As a result of the P-CSCF failure or malfunction, any UE that had previously registered with the P-CSCF can no longer receive terminating calls that are set up through the IMS network. It has been suggested to register each UE with at least two distinct P-CSCF nodes of the IMS network. This solution mitigates the impact of the described problem to some degree as the probability that both P-CSCF fail at the same time is low. Therefore the UE is likely reachable through at least one of the P-CSCF nodes with which it registered at any point in time. However, such a solution is not foreseen as part of the VoLTE specification, and therefore it is not applicable to one of the main applications in which the above problem arises.
A further suggestion would be to let the PDN Gateway node of the radio access network, which connects to the P-CSCF node of the IMS network, regularly check the availability of the P-CSCF node. This may be achieved by sending “ping” signals at regular intervals. If a failure is detected, the Packet Gateway node sends addresses of available alternative P-CSCF nodes to all UEs which were registered with the unavailable P-CSCF. These UEs may then re-register themselves at one of the alternative P-CSCF nodes. This solution involves potential mass signalling in the radio access network in case of a P-CSCF failure. Moreover, it requires all UEs that were registered with the unavailable P-CSCF node to re-register, while only a fraction of the UE's may have experienced the above mentioned problem. Indeed the problem only arises when a call handled by the IMS network should terminate at a UE that was registered with the unavailable P-CSCF node.
A third option is to force each UE to frequently re-register with a P-CSCF node. If re-registration with the previously used P-CSCF node fails, the UE will register with an alternative, available P-CSCF node. However, this solution causes the batteries of the terminals implementing the UE to drain quickly, as they typically use a cellular network access.